1. Field of the Invention
The present invention generally relates to an optical add-drop multiplexer (referred to as an OADM hereinafter) installed in an optical wavelength-division multiplexed (WDM) communications network for extracting a specific wavelength from an optical WDM signal and simultaneously complementing the optical WDM signal with an optical signal component having a wavelength identical to that of the, extracted wavelength.
2. Description of the Related Art
Optical WDM communications networks are normally used as bulk-sized communications networks. In such networks, a plurality of light signals are multiplexed and transmitted along a single optical fiber line. More specifically, since different optical wavelengths of their respective light signals are assigned to different receivers in the network, multiple-to-multiple communications arrangements are made possible.
Referring to FIG. 4, U.S. Pat. No. 5,822,095 discloses an OADM apparatus for use in a WDM network. The wavelength components xcex1 to xcexn of an input n-wavelength signal are received through an input optical fiber 1xe2x80x2 and pass through a circulator 3xe2x80x2 to an optical bandpass filter 4xe2x80x2, which allows a specific wavelength xcex1 to pass but rejects the other wavelengths xcex2 to xcexn. While the rejected wavelengths xcex2 to xcexn are returned via the optical fiber 5xe2x80x2, the specific wavelength xcex1 transmits through another optical fiber 9xe2x80x2 and another circulator 8xe2x80x2, and then is dropped via an output optical fiber 6xe2x80x2. Meanwhile, a second signal component having wavelength xcex1 is introduced through another input optical fiber 7xe2x80x2 and transmits through the optical circulator 8xe2x80x2, through the optical fiber 9xe2x80x2 and through the optical bandpass filter 4xe2x80x2, whereupon it enters the optical fiber 5xe2x80x2 and is added to the rejected wavelengths xcex2 to xcexn. A resultant sum signal of the full set of wavelengths xcex1 to xcexn is passed through the optical circulator 3xe2x80x2 and is transmitted from the OADM apparatus via an output optical fiber 2xe2x80x2.
Referring to FIG. 5, U.S. Pat. No. 5,926,300 also discloses an OADM apparatus, which comprises a first and a second optical circulators 1, 2, a first and a second wavelength reflecting fiber gratings 3, 4 and an isolator 5 connected between the first and the second wavelength reflecting fiber gratings 3, 4. The isolator 5 is provided for preventing a light signal from being reflected back and being amplified between the first and the second wavelength reflecting fiber gratings 3, 4.
However, the above-mentioned OADM apparatus comprise optical circulators, fiber gratings or an isolator and thus include many complicated optical components, which can result in the apparatus being complex to construct and high in cost.
Therefore, an object of the present invention is to provide an improved OADM apparatus which is easy to construct and low in cost.
An OADM apparatus in accordance with the present invention includes an input/output channel, a drop channel, an add channel and a filter. Light signals having a plurality of wavelengths pass through the input/output channel, one specific wavelength of the light signals passes through the filter and the drop channel and goes into a drop fiber, all other wavelengths are reflected back. Another light signal having the specific wavelength passes through the add channel and the filter and combines with the reflected wavelengths of the light signals, and the combined wavelengths pass through the input/output channel and go into the output fiber.
Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of a preferred embodiment of the present invention with attached drawings, in which: